The present invention relates to a mass or mass to charge ratio selective ion trap. The preferred embodiment relates to ion guiding and trapping systems and methodology for use in mass spectrometry systems.
It is well known that the time averaged force on a charged particle or ion due to an AC inhomogeneous electric field is such as to accelerate the charged particle or ion to a region where the electric field is weaker. A minimum in the electric field is commonly referred to as a pseudo-potential well or valley. Correspondingly, a maximum is commonly referred to as a pseudo-potential hill or barrier.
Paul traps, also known as 3D ion traps, are designed to exploit this phenomenon by causing a pseudo-potential well to be formed in the centre of the ion trap. The pseudo-potential well is then used to confine a population of ions. Due to its symmetric nature the 3D ion trap acts to confine ions to a single point in space as shown in FIG. 1A. However, the mutual repulsion between ions of identical polarity in addition to the non-zero kinetic energy of the confined ions lead to the ions occupying a spherical volume at the centre of the ion trap as illustrated in FIG. 1B.
There is a finite space charge capacity for any ion confining device beyond which its performance begins to degrade and where ultimately the device cannot hold any further charges. For example, overfilling an ion trap leads to a loss of mass resolution and of mass accuracy, a result of the electric field becoming distorted by the presence of the large number of charges being focussed into close proximity. It is generally the case that the space charge limit for storage of ions is significantly greater than the spectral or analytical space charge limit which is the maximum number of ions which can be confined whilst retaining a given mass resolution and mass accuracy.
For mass spectrometry applications it is necessary to detect the mass to charge ratio (m/z) of the confined ions. For example, ions may be ejected in a mass selective manner towards an ion detector (although many other detection methods exist). There are several known methods of ejecting ions either resonantly or non-resonantly to achieve this goal.
It is often necessary to introduce gas into ion trapping devices. The gas may be used for cooling purposes or ion fragmentation via Collision Induced Decomposition (“CID”). Ion Mobility Separation (“IMS”) has also been performed either with a static volume of gas or with a flow of gas. The use of pulsed gas valves to introduce gas into ion traps is also known.
Recently, there has been increased interest in 2D or Linear Ion Traps (“LIT”) because of the increased volume which the confined ions are able to occupy. Linear ion traps allow a greater number of ions, or more correctly a greater number of charges, to be confined and then detected. Such ion traps are generally based on multipolar RF ion guides such as quadrupoles, hexapoles or octopoles. A pseudo-potential well is formed within the rod set ion trap around the central axis of the ion guide so that ions are confined radially within the ion trap. The ions are normally confined axially using DC fields although methods of using RF fields to axially confine ions are also known.
The radial pseudo potential of a 2D ion trap acts to focus the confined ions to a line through the central axis of the ion trap as shown in FIG. 1C. In a similar manner to 3D ion traps, ions confined within a 2D ion trap will in practice be spatially distributed and thus occupy an elongated cylindrical volume as shown in FIG. 1D.
Ion ejection has been demonstrated both radially and axially using 2D ion traps by resonantly exciting the ions within the confining radial pseudo potential. Radial ejection has been achieved by allowing the ions to resonate until their radial excursions reach the quadrupole electrodes at which point they pass through narrow slots in the electrodes. Axial ejection has been achieved by resonantly exciting the ions into the naturally occurring fringing fields which exist at the exit of a quadrupole at which point it is possible for the ions to gain sufficient axial kinetic energy to overcome the confining DC barrier. Both of these methods are inherently non-adiabatic in nature and lead to large ejection energies and large energy spreads which makes them generally unsuitable for coupling with other devices such as other mass analysers.
Another form of axial ejection from a 2D ion trap is known and comprises superimposing an axial harmonic DC potential upon a radial confining RF of an ion guide. Such approaches are schematically represented in FIGS. 2A-C.
FIG. 2A shows a 2D ion trap comprising a series of annular electrodes which coaxially encompass a quadrupole rod set. RF voltages are applied to the rod set electrodes in order to cause ions to be radially confined. DC voltages are applied to the annular electrodes to produce an axial DC potential within the rod set.
FIG. 2B shows a 2D ion trap comprising an RF quadrupole rod set with additional vane electrodes placed on the ground planes which are used to provide an axial DC potential.
FIG. 2C shows a 2D ion trap comprising an axially segmented RF quadrupole rod set. Different DC voltages may be applied to each segment in order to provide an axial DC potential.
With respect to the 2D ion traps shown in FIGS. 2A-2C, the DC potential which is applied in the axial (z) direction is given by Eqn. 1:Uz(t)=(a+b·cos(Ωt))·z2  (1)where b is the electric field constant of the axial quadratic potential, a is the amplitude and Ω is the frequency of the modulation of the axial potential.
                                                                        E                z                            =                            ⁢                                                ⅆ                                                            U                      z                                        ⁡                                          (                      t                      )                                                                                        ⅆ                  z                                                                                                        =                            ⁢                              2                ⁢                                                                  ⁢                                                      (                                          a                      +                                                                        b                          ·                          cos                                                ⁢                                                                                                  ⁢                                                  (                                                      Ω                            ⁢                                                                                                                  ⁢                            t                                                    )                                                                                      )                                    ·                  z                                                                                        (        2        )                                                                    z              ¨                        +                                          ω                2                            ⁢              z                                =                                    F              ·              cos                        ⁢                                                  ⁢                          (                              Ω                ⁢                                                                  ⁢                t                            )                                      ⁢                                  ⁢                  ω          =                                                    2                ⁢                                                                  ⁢                aq                            m                                      ⁢                                  ⁢        and        ⁢                                  ⁢                  F          =                                    2              ⁢                                                          ⁢              bq                        m                                              (        3        )                                          z          ⁢                                          ⁢                      (            t            )                          =                              F                                          ω                2                            -                              Ω                2                                              ⁢          sin          ⁢                                          ⁢                      (                                          Ω                ⁢                                                                  ⁢                t                            +              ϕ                        )                                              (        4        )            